The overall aim of this project is to continue development of PET cameras whose geometry is optimized for detecting breast cancer or axillary node involvement. These post-x-ray mammography tools would determine whether suspicious structures observed in mammograms have the increased metabolism associated with breast cancers, and can image the axilla to determine the extent of axillary node involvement. The instrumentation proposed has the potential to provide a cost effective, non-invasive alternative to biopsy as well as accurate information on axillary node involvement with significantly higher sensitivity than existing imaging techniques. The proposed technique relies on the fact that FDG is an excellent tracer for breast cancer with >90% specificity and selectivity, as measured with conventional PET imaging. The proposed instruments consist of PET detector modules placed in close proximity to the breast (similar to a mammography unit) or the axilla (similar to a small diameter PET ring). These geometries improve the sensitivity and the spatial resolution significantly compared to a conventional PET camera (the sensitivity increase is a factor of 4-30 for the breast and a factor of 1-10 for the axilla), and so allow rapid identification of cancerous lesions and axillary involvement for structures down to 5 mm in size with a small (<1 mCi) injected dose of FDG. The first funding period of this project on development of the base technology for a high performance PET detector module consisting of a large number of small LSO scintillator crystals, each coupled on one end to a photomultiplier tube (which provides a timing pulse and energy discrimination) and on the other end to an individual silicon photodiode (which identifies the crystal of interaction and measures the depth of interaction within the crystal). The present project focuses on construction of two PET cameras, development of reconstruction algorithms for these unique camera geometries, characterization of the imaging properties with phantoms, and preliminary evaluation of the imaging properties in patients. The small detector volume reduces the camera cost by a factor of ten compared to a conventional PET camera. When combined with the lower amount of radio-pharmaceutical needed, this development could reduce the cost of a patient examination significantly. These cameras are designed specifically for breast cancer, but the detector module developed could also be incorporated into high resolution PET cameras with conventional geometries. These designs, when complete, will be offered to private industry to incorporate into a clinical instrument.